Dudes that happen to be gay, but aren't flamboyant at all. Prefer to go drink beer at a buddy's place, rather than go to the gay bar and pay for overpriced cocktails and listen to bouncy, campy club songs.
A bromo is not conceited in his looks, but is still put together decently, groomed but not over plucked.
Not to be confused by bromance.
i.e., I really like my guy to be a bromo, not overly gay, but gay just where it counts.